Immersion agent coupling device and method for coupling an optical waveguide

ABSTRACT

A transparent elastomer (e.g. silicon rubber) is used as an immersion agent ( 1 0) for durable and low-attenuation connection of an optical waveguide ( 2 ) to the conductive structures of an optical component ( 6 ). The matrix of said elastomer contains a non-cross-linked proportion of a liquid phase (e.g. silicon softeners). The same kind of immersion agent ( 10 ) is used in the coupling system. The distance between the end face of the optical waveguide ( 2 ) and the coupling surface of the component ( 6 ) ranges from approximately 2 μm to 20 μm. The volume of the immersion agent ( 10 ) present at the point of coupling ( 8 ) is less than 5 μl.

DESCRIPTION

[0001] Immersion agent, coupling arrangement and coupling method foroptical waveguides

[0002] The invention relates to an immersion agent, a couplingarrangement and a method for coupling at least one optical waveguide(LWL) to an optical component (chip) or for connecting an opticalwaveguide to an optical component.

[0003] An optical coupling arrangement is used to inject light or couplelight between two optical waveguide end faces, for example between theend face of an optical waveguide fiber composed of a core and cladding,and the opposite end face of a waveguide structure present on a chip.Such coupling arrangements are used, for example, in optical filterswhich operate according to the phased array principle. These have aninjection face in which light enters at a specific point, the outputwavelength of the optical filter depending on the geometric position ofthe injection point. Optical filters operating according to the phasedarray principle are used in particular as multiplexers or demultiplexersin the field of optical telecommunications transmission as they have lowinsertion attenuation and high crosstalk suppression.

[0004] German patent application DE 44 22 651.9 describes what isreferred to as a phased array filter whose central wavelength is definedby positioning the optical waveguide fiber which injects the light intothe chip waveguide structure, and can thus be precisely aligned. This isdone by displacing the waveguide end faces in relation to one another.

[0005] It has already been proposed to change the position of the endface of the optical waveguide with respect to the injection face of thechip by virtue of the fact that an element with a variable length isfitted with the fiber and the latter is thus displaced in parallel withthe direction of expansion of the element with a variable length.

[0006] In order to implement optical coupling between an opticalwaveguide, an optical waveguide fibre or fiber array and an opticalelement containing active and/or passive elements, the opticalwaveguides must be held in a defined position with respect to theinjection face of the chip and connected to the corresponding waveguidestructure. This is usually carried out by directly bonding the fiberends to the chip. However, in the abovementioned cases, direct bondingor welding of the fiber ends to the chip is not desired as this wouldchange the necessary relative movement between the fiber and the chip.In order to improve the insertion attenuation and reduce the powerfluctuations, hitherto an immersion agent has been introduced betweenthe fiber and the chip. In particular a stable gel, for example anadditional crosslinked silicone rubber is used as the immersion agent.The silicone rubber is composed here of two components whose mixingratio is 1:1 so that the rubber hardens completely after itsintroduction. The hardening is considered necessary in order to preventthe immersion agent flowing away.

[0007] As an enclosed volume is formed between the fiber and its fibermount on the one hand and the chip on the other, fractures or vacuoles(small vacuum bubbles) which considerably increase the insertionattenuation of the component are formed in the immersion gel duringcooling as a result of shrinkage. In addition, the formation offractures is also promoted by the abovementioned relative movementbetween the fiber and chip.

[0008] The invention is therefore based on the object of makingavailable an optical coupling between an optical waveguide, for examplea fiber or a fiber array, and an optical component/chip which has a lowinsertion attenuation.

[0009] In order to achieve this object, an immersion agent for couplingoptical waveguides to an optical chip is characterized in that atransparent elastomer serves as the immersion agent, the tensile strainat break of the elastomer being greater than 300% and its modulus ofelasticity having a value which is smaller than 200 N/cm². The immersionis therefore adjusted to be so soft that in the case of coolingsufficient gel continues to flow out of the edge regions of theimmersion and stresses which lead to fractures and vacuoles cannot buildup. On the other hand, the immersion material is not so liquid that itcan flow out of the volume between the optical waveguide and chip, thusensuring that the immersion material remains in this intermediate spaceduring the entire service life of the component.

[0010] One advantageous configuration of the immersion agent accordingto the invention is characterized in that the matrix of the immersionagent contains a noncrosslinked proportion of a liquid phase. Bycorrespondingly adjusting the mixing ratio, for example of atwo-component immersion agent, it is thus possible to adjust theimmersion agent so that it is soft in the desired way.

[0011] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that the immersion agentcontains a liquid phase proportion of 1 to 10% of an immersion liquidwith a low vapor pressure. The addition of a liquid phase proportion isa simple alternative to generating an immersion agent with theproperties mentioned at the beginning.

[0012] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that an immersion oil ora softener which is selected as a function of the elastomer is used asthe immersion liquid, the immersion agent being easily adjustable byadding the immersion oil.

[0013] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that the immersion agentis silicone rubber, and in that the immersion liquid is a siliconesoftener, in particular silicone oil. As the silicone rubber has apronounced degree of adhesiveness even in the state in which it is notcompletely crosslinked, stability of the coupling of the component isensured over its entire service life.

[0014] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that the immersion agentis epoxy acrylate, and in that the immersion liquid is an epoxy acrylatesoftener, in particular polyisobutylene.

[0015] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that the immersion agentis urethane acrylate, and in that the immersion liquid is a urethaneacrylate softener, in particular polyisobutylene.

[0016] The two last-mentioned immersion agents are also referred to asradiation crosslinked elastomers which constitute an advantageousalternative to silicone rubber. These immersion agents can also have theproperties required of the immersion agent according to the invention.

[0017] A further advantageous configuration of the immersion agentaccording to the invention is characterized in that the immersion liquidor the softener is an aliphatic or aromatic oil so that a series ofsofteners is available, it being possible to select the specificsoftener according to the respective peripheral conditions.

[0018] A further advantageous refinement of the immersion agentaccording to the invention is characterized in that the silicone rubberis composed of two components, the mixing ratio of the components being0.5:1 to 0.9:1. In the case of silicone rubber, for example in the caseof the silicone rubber WACKER SilGel® 612 from Wacker-Chemie GmbH, amixing ratio of the components of 1:1 is recommended in order to obtaincomplete crosslinking of the silicone rubber. If this mixing ratio ischanged in the way stated, a silicone rubber is obtained which has thesoftness required according to the invention.

[0019] A further advantageous refinement of the immersion agentaccording to the invention is characterized in that the immersion agenthas an adhesive force per unit area which is greater than half thematerial breaking stress of the elastomer, it being advantageouslyensured that the immersion agent does not become detached from thecoupling face of the fiber or of the chip when a shrinkage processoccurs.

[0020] A further advantageous refinement of the immersion agentaccording to the invention is characterized in that the transformationpoint/glass transition point of the immersion agent T_(g) is below 0° C.This advantageously ensures that the immersion agent has the desiredelastic properties over the entire operating temperature range of thecoupling.

[0021] A further advantageous refinement of the immersion agentaccording to the invention is characterized in that the refractive indexof the immersion agent has a value between n=1.3 and n=1.7, inparticular between n=1.4 and n=1.5. By thus selecting the refractiveindex, the coupling arrangement is adapted in an optimum way to therefractive indices of the fibers or of the chip, and the couplingattenuation is correspondingly reduced.

[0022] In order to achieve the abovementioned object, an arrangement forcoupling optical waveguides, for example optical fibers or a fiberarray, to an optical component/chip is characterized in that

[0023] (a) an immersion agent of the abovementioned type is used, inthat

[0024] (b) the distance between an end face of the optical waveguide anda coupling face of the chip is 2 μm to 20 μm, and in that

[0025] (c) the volume of the immersion agent applied to the couplingpoint is less than 5 μl.

[0026] In trials it has been shown that both the properties of theimmersion agent and the spatial conditions in the region of the couplingarrangement, that is in particular the distance between the opticalwaveguides and the coupling face of the chip and the volume of immersionagent, influences the quality of the coupling and the service life ofthe coupling arrangement. The formation of fractures or vacuoles canadvantageously be reduced further by this arrangement if the distancebetween a coupling face of the optical waveguide and the coupling faceof the chip is between 2 μm and 20 μm, and if the volume of theimmersion agent applied to the coupling point is below 5 μl. If thedistance between the coupling faces is less than 2 μm, there is the riskof the coupling faces touching one another or of the friction betweenthe coupling faces being so large that the immersion agent becomesdetached from the coupling faces. If the distance is greater than 20 μm,the attenuation increases considerably as a result of the immersionagent. On the other hand, compliance with these parameters ensures thatif a shrinkage process occurs sufficient immersion agent can flow fromthe outside into the space between the coupling faces so that theformation of vacuoles is avoided.

[0027] A further advantageous refinement of the arrangement according tothe invention in which the optical waveguide fiber is arranged forcoupling in a ferrule is characterized in that the fiber is held in aferrule, and in that the ferrule has a reduced diameter at its end side,and in that the coupling face of the chip is also reduced. This ensuresthat the distance from the edge region of the volume between the twocoupling faces and the center of this region becomes smaller so that ifa shrinkage process occurs the gel can better and more quicklycompensate stresses which possibly occur, in order to avoid fracturesand the formation of vacuoles.

[0028] A further advantageous refinement of the arrangement according tothe invention is characterized in that the coupling device is surroundedby a sealing compound which has the same components as the immersionmaterial but is adjusted so as to be harder, and is in particularcompletely crosslinked. By filling the coupling point with castingmaterial, mechanical shock stresses and vibrations are then onlytransmitted to the coupling arrangement in an attenuated fashion. If thesealing compound is made of the same immersion material as the immersionmaterial for the coupling, for example also of silicone rubber, chemicalcompatibility between the two materials is advantageously obtained,which differ only in the degree of hardening or in an addition ofsoftener in the case of the immersion agent at the coupling point. Inaddition, the coefficients of expansion of the two materials areessentially identical so that when there are expansion or shrinkageprocesses no additional compressive forces or tensile forces are exertedon the immersion material at the coupling point. Finally, the elastomerimmersion material is advantageously protected against flowing away bythe sealing compound.

[0029] In order to achieve the abovementioned object, a method forcoupling optical waveguides, for example optical fibers or a fiberarray, to an optical chip is characterized in that

[0030] (a) an optical waveguide or a ferrule connected thereto is movedto a distance of 2 to 20 μm away from a coupling face of the chip, inthat

[0031] (b) an immersion material of the abovementioned type is prepared,in that

[0032] (c) the immersion material is dispensed at the coupling point toa quantity of approximately 5 μl, and in that

[0033] (d) the immersion material is allowed to harden in accordancewith the mixing ratio.

[0034] Maintaining the spatial relationship between the coupling face ofthe optical waveguide and the coupling face of the chip, that is to sayselecting the distance between these coupling faces, ensures, without alarge degree of expenditure during the coupling method, that during ashrinkage process or expansion process, a considerable formation offractures or vacuoles occurs even while the coupling arrangement isoperating. The coupling method itself is as simple here as in the priorart so that no additional expenditure is necessary to implement theinvention. Maintaining a relatively large distance between the couplingfaces also contradicts the previous practice according to which it wasattempted to arrange the two coupling faces as close to one another aspossible in order to improve the coupling of the light beam from theoptical waveguide into the chip. However, this coupling is surprisinglyworse if the difference between the coupling faces is too low becauseother mechanical and stress influences then act to the effect that theimmersion material can no longer bring about adequate optical couplingbetween the fiber and the chip.

[0035] A further advantageous refinement of the method according to theinvention is characterized in that, after the hardening of the immersionmaterial, a sealing compound which has the same components as theimmersion material but is adjusted so as to be harder, and is inparticular completely crosslinked, is cast around the coupling point. Asthe same starting material is used for sealing the component and also asan immersion material for the coupling, this material merely has to beadjustable to different hardnesses in order to be suitable for thispurpose. By using the same starting material, not only theabovementioned advantages are obtained but also stockholding is improvedas it is not necessary to keep different materials for the two purposesof use in stock.

[0036] A further advantageous refinement of the method according to theinvention is characterized in that the immersion material or the sealingcompound is allowed to harden at normal operating temperature. Thisadvantageously avoids the situation in which the material hardens at atemperature which differs considerably from the normal operatingtemperature so that the component is already subjected to stresses whenit is moved from the production site to the location where it is to beused. These stresses then occur in addition to the “normal” stressloading which occurs during the operation of the component. If theoriginal hardening of the component has taken place at the normaloperating temperature, the formation of fractures and vacuoles will alsobe less in the cases at the operating site if the component has in themeantime been located in environments with different temperatures.

[0037] A further advantageous refinement of the method according to theinvention is characterized in that the immersion material whose matrixcontains a proportion of a liquid phase is manufactured in that acompletely hardened immersion material which is applied to the couplingpoint is treated with a softener. As a result of this method, alreadyexisting coupling arrangements can advantageously be protected againstfuture fractures and the formation of vacuoles. In other words, byapplying a suitable oil or another softener liquid to a hardenedimmersion material the immersion material is placed in a state which issuitable for the purposes according to the invention.

[0038] A further advantageous refinement of the method according to theinvention is characterized in that aliphatic or aromatic oils are usedas immersion oils. These oils are also suitable for the subsequentconditioning of the immersion material for the purposes of the inventionso that the desired purpose can be achieved without relatively largeadditional expenditure.

[0039] Exemplary embodiments of the invention will now be described withreference to the appended drawing in which a coupling arrangement forcoupling an optical waveguide fiber to an optical component/chip isillustrated schematically.

[0040] The figure shows a coupling arrangement between an optical fiber2 which is anchored in a ferrule 4 and an integrated optical component,that is to say a chip 6 with waveguides (not shown) to be coupled. Animmersion agent or immersion gel 10 whose volume of approximately 5 μlor less fills the intermediate space between the end side of the fiber 2or the ferrule 4 and the coupling face on the chip 6 as well as an edgeregion surrounding this coupling region is provided at the couplingpoint 8. The parts of the coupling arrangement are sealed by a sealingcompound 12 in a housing 14. The immersion agent 10 has the followingproperties: It has a refractive index between 1.3 and 1.7, for examplebetween 1.4 and 1.5.

[0041] It is a transparent elastomer with a tensile strain at break ofover 300%. The tensile strain at break can easily have values of 1000%.Given the coupling arrangements mentioned at the beginning, which permita movement between the fiber and the chip, lateral displacements betweenthe two coupling faces of 30 to 40 μm occur. Given a distance betweenthe two coupling faces of 3 μm, a tensile strain at break ofapproximately 1000% occurs.

[0042] The immersion material has a modulus of elasticity below 200N/cm², preferably below 100 N/cm².

[0043] The immersion material has a transformation point/glasstransition point T_(g) below 0° C.

[0044] The immersion material has the required softness or adhesivenessby virtue of the fact that, for example in the case of silicone rubber,the mixing ratio of the two components of 0.5:1 to 0.9:1 is selected.Alternatively, a liquid phase proportion of softener or immersion oil toa quantity of 0% to 10% of the quantity of the immersion material can beadded, the immersion liquid having a low vapor pressure.

[0045] The immersion agent has an adhesive force per unit area incomparison with quartz glass of at least half the material breakingstress of the elastomer.

[0046] The sheet thickness of the immersion material between thecoupling faces is between 2 μm and 20 μm, while the overall length ofthe immersion material per fiber coupling is below 5 μm.

[0047] The material of the sealing compound is the same startingmaterial as the immersion material, but the sealing compound iscompletely hardened. The sealing compound advantageously protects theelastomer immersion material against flowing away.

1. An immersion agent for coupling at least one optical waveguide to an optical component, characterized in that the immersion agent is a transparent elastomer whose tensile strain at break is greater than 300% and whose modulus of elasticity is smaller than 200 N/cm².
 2. The immersion agent as claimed in claim 1, characterized in that the matrix of the immersion agent contains a noncrosslinked proportion of a liquid phase.
 3. The immersion agent as claimed in claim 1, characterized in that the liquid phase proportion is 1 to 10% of an immersion liquid with a low vapor pressure.
 4. The immersion agent as claimed in claim 1 or 2, characterized in that the immersion liquid is immersion oil or a softener which is selected as a function of the elastomer.
 5. The immersion agent as claimed in one of claims 1 to 3, characterized in that the immersion agent is silicone rubber, and in that the immersion liquid is a silicone softener, in particular silicone oil.
 6. The immersion agent as claimed in one of claims 1 to 3, characterized in that the immersion agent -is epoxy acrylate, and in that the immersion liquid is an epoxy acrylate softener, in particular polyisobutylene.
 7. The immersion agent as claimed in one of claims 1 to 3, characterized in that the immersion agent is urethane acrylate, and in that the immersion liquid is a urethane acrylate softener, in particular polyisobutylene.
 8. The immersion agent as claimed in one of the preceding claims, characterized in that the softener is an aliphatic or aromatic oil.
 9. The immersion agent as claimed in claim 4, characterized in that the silicone rubber is mixed from two components, the mixing ratio of the components being 0.5:1 to 0.9:1.
 10. The immersion agent as claimed in one of the preceding claims, characterized in that the immersion agent has an adhesive force per unit area which in comparison with glass is greater than half the material breaking stress of the elastomer.
 11. The immersion agent as claimed in one of the preceding claims, characterized in that the transformation point/glass transition point of the immersion agent T_(g) is less than 0° C.
 12. The immersion agent as claimed in one of the preceding claims, characterized in that the refractive index of the immersion agent is 1.3 to 1.7, in particular 1.4 to 1.5.
 13. An arrangement for coupling at least one optical waveguide to an optical component, characterized in that (d) an immersion agent as claimed in one of claims 1 to 12 is used, in that (e) the distance (d) between an end face of the optical waveguide and a coupling face of the component is 2 μm to 20 μm, and in that (f) the volume of the immersion agent applied to the coupling point is less than 5 μl.
 14. The arrangement as claimed in claim 13, the optical waveguide fiber being arranged for coupling in a ferrule, characterized in that the ferrule has a reduced diameter at its end side, and in that the coupling face of the component is also reduced.
 15. The arrangement as claimed in claim 13 or 14, characterized in that the coupling device is surrounded by a sealing compound which has the same components as the immersion material but is adjusted so as to be harder, and is in particular completely crosslinked.
 16. A method for coupling at least one optical waveguide to an optical component, characterized in that (e) an optical waveguide or a ferrule connected thereto is moved to a distance of 2 to 20 μm from a coupling face of the component, in that (f) an immersion material as claimed in one of claims 1 to 12 is prepared, in that (g) the immersion material is deposited at the coupling point to a quantity of approximately 5 μl, and in that (h) the immersion material is allowed to harden in accordance with the mixing ratio.
 17. The method as claimed in claim 15 or 16, characterized in that, after the hardening of the immersion material, a sealing compound which has the same components as the immersion material but is adjusted so as to be harder, and is in particular completely crosslinked, is cast around the coupling point.
 18. The method as claimed in claim 17, characterized in that the immersion material or the sealing compound is allowed to harden at normal operating temperature.
 19. The method as claimed in claim 18, characterized in that the immersion material whose matrix contains a proportion of a liquid phase is manufactured in that a completely hardened immersion material which is applied to the coupling point is treated with a softener.
 20. The method as claimed in claim 19, characterized in that aliphatic or aromatic oils are used as immersion oils. 